A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in two articles by T. H. Van Steenkiste et al. The first was entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62–71, Jan. 10, 1999 and the second was entitled “Aluminum coatings via kinetic spray with relatively large powder particles”, published in Surface and Coatings Technology 154, pp. 237–252, 2002. The articles discuss producing continuous layer coatings having high adhesion, low oxide content and low thermal stress. The articles describe coatings being produced by entraining metal powders in an accelerated gas stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate surface. The particles are accelerated in the high velocity gas stream by the drag effect. The gas used can be any of a variety of gases including air, nitrogen, argon, neon or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation upon striking the substrate. Thus, it is believed that the particle velocity must exceed a critical velocity and be high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet gas temperature was increased. Increasing the inlet gas temperature decreases its density and thus increases its velocity. The velocity varies approximately as the square root of the inlet gas temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. The critical velocity is dependent on the material of the particle and the material of the substrate. Once an initial layer of particles has been formed on a substrate subsequent particles bind not only in the voids between previous particles bound to the substrate but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the main gas stream because the temperature of the particles is always below their melting temperature.
Kinetic spray technology would greatly reduce the cost of manufacturing if it could be used to coat materials covered by plastic-type material without requiring prior removal of the plastic material. One area of special concern is flexible electrical circuitry. In these systems electrical conductors, typically ribbon wire, are covered in a plastic-type coating to protect them and to electrically isolate them from each other. Other plastic covered substrates of interest include ceramics. In the present specification and claims the term plastic-type material is meant to designate not only true plastics but also polyurethanes, polymers, nylons, rubbers, and elastomers. These coverings are relatively soft compared to the metals and ceramics that typically make up the underlying substrate. As mentioned, one common manufacturing environment that could benefit from the technology is the flexible circuit wiring area. This wiring is the typical plastic covered ribbon type wiring found in computers, automobiles and other electrical systems. Currently, when one desires to manufacture an electrical connection point or solderable pad somewhere along the flexible circuit it is necessary to remove the outer plastic covering in some manner prior to making the connection. Typically, this is done by laser ablation, using a punch wheel, or milling. The exposed wiring is then cleaned and finally, electroplated. These steps are very time consuming, require a large manufacturing footprint, and generate waste problems. It would be advantageous to develop a method for applying a kinetic spray coating onto a surface that is covered in a plastic-type material without requiring prior removal of the plastic-type material. The kinetic sprayed coating can serve as an electrical contact or solder point.